EU negotiators will resume controversial trade talks with the US on Monday amid claims that multinational companies have jumped the gun in advance of any agreement to import goods that are currently banned – including genetically modified crops and chemically washed beef – into European markets.

The accusation comes as the European commission faces intense pressure to abandon the controversial talks, which critics say will undermine food safety, environmental standards and job security. More than three million people in Europe have signed a petition against the deal while an estimated 250,000 people marched in Berlin last weekend against the proposals.

John McDonnell, the shadow chancellor, has described TTIP as “toxic” and resulting in a huge transfer of powers to Brussels and corporate interests that will bring about a form of “modern-day serfdom”.

The EU commission wants to sign a trade accord before next spring with the US to lower trade barriers and boost growth. EU leaders argue that a TTIP deal would create a free trade zone covering 800 million people and act as a counterweight to China’s growing economic power. Brussels has predicted it would add $92bn to the EU’s $18.46tn GDP.

More than two dozen EU negotiators will meet US officials in Miami to discuss harmonising regulations alongside new rules for public procurement.

Dearden says that according to a report in the newsletter Washington Trade Daily, Levie told a conference held by US free market thinktank the Cato Institute that genetically modified crops and chemically washed beef carcasses were being allowed into the EU ahead of a deal.

The outcome of TTIP talks on public contracts, which will govern how US firms can bid for work in the health service, schools and other areas of the public sector, is also expected to prove controversial. Unions and anti-poverty campaign groups have warned of backdoor privatisations that will undermine the public sector ethos of health and education provision.

EU trade commissioner Cecilia Malmström has rejected concerns that an agreement will result in US health and education businesses undercutting European operators to secure contracts.

A spokesman for the EU commission said that Malmström’s team intends to secure an agreement to allow public bodies to support sustainable development that safeguard labour and environmental concerns. He said there would also be “thorough technical discussions” covering the sale of cars, chemicals, pharmaceuticals, medical devices, engineering, pesticides, cosmetics and textiles.

“Our objective is to define more precisely what can be achieved concretely in terms of greater regulatory compatibility,” he said.

Malmström has also sought to head off criticism that a TTIP deal will allow corporations to win compensation from governments if they suffer from public policy changes via a new “investment court”, though this will not be discussed in Miami.

Levie, who is expected to attend the Miami talks, told the Cato Institute conference that neither side wants to reach anything less than a comprehensive economic agreement. He conceded the deal could founder on resistance from the US to include financial services in the deal and Washington’s reluctance to open local and state procurement to bids from EU businesses.

In defence of Levie’s comments on food, the European commission said Levie was referring to separate decisions to allow chemically washed beef from the US into European markets that were agreed in 2013 “following favourable advice from the European Food Safety Authority (EFSA) on its use”.

The spokesman said: “This issue bears no relation with the ongoing negotiations. Our stance on not lowering our standards in the framework of TTIP or any other trade negotiation is clear.”

Dearden said he remained concerned that to harmonise regulations, negotiators were engaged in a “race to the bottom”.

“TTIP is already letting big business interests dictate our laws for the worse. This week an EU negotiator has let slip that negotiations on TTIP have helped speed up entry of GMOs and chemically washed beef into the EU market. In our briefing released today we found an example of US officials bullying the EU into dropping plans to ban 31 dangerous pesticides with ingredients that have been shown to cause cancer.

“Just imagine what will happen when TTIP actually comes into effect. Even the most optimistic of citizens must surely doubt the EU’s good intentions on TTIP after hearing how TTIP is already letting big business take over our legislative system. TTIP is about forcing governments to see the whole of society from the viewpoint of big business. Every regulation which is important to society, workers’ rights or environmental protection becomes simply an obstacle to profit.”

• This article was amended on 3 November 2015. An earlier version said that according to a report in the Washington Trade Daily, the EU’s chief trade counsellor, Damien Levie, told a conference that EU member states had been stepping up case reviews and approving new genetically modified organisms with five new products approved so far. The WTD has now accepted that Levie did not mention GMOs in his presentation.